End-to-End Distance Probability Distributions of Dilute Poly(ethylene oxide) in Aqueous Solution

We introduce a powerful, widely applicable approach to characterizing polymer conformational distributions, specifically the end-to-end distance distributions, P(R-ee), accessed through double electron-electron resonance (DEER) spectroscopy in conjunction with molecular dynamics ( MD) simulations. The technique is demonstrated on one of the most widely used synthetic, disordered, water-soluble polymers: poly(ethylene oxide) (PEO). Despite its widespread importance, no systematic experimental characterization of PEO's R-ee conformational landscape exists. The evaluation of P(R-ee) is particularly important for short polymers or (bio)polymers with sequence complexities that deviate from simple polymer physics scaling laws valid for long chains. In this study, we characterize the R-ee landscape by measuring P(R-ee) for low molecular weight (MW: 0.22-2.6 kDa) dilute PEO chains. We use DEER with end-conjugated spin probes to resolve R-ee populations from similar to 2-9 nm and compare them with full distributions from MD. The P(R-ee)'s from DEER and MD show remarkably good agreement, particularly at longer chain lengths where populations in the DEER-unresolvable range (<1.5 nm) are low. Both the P(R-ee) and the root-mean-square <(R)over bar>(ee) indicate that aqueous PEO is a semiflexible polymer in a good solvent, with the latter scaling linearly with molecular weight up to its persistence length (l(p) similar to 0.48 nm), and rapidly transitioning to excluded volume scaling above l(p). The (R) over bar (ee) scaling is quantitatively consistent with that from experimental scattering data on high MW (>10 kDa) PEO and the P(R-ee)'s crossover to the theoretical distribution for an excluded volume chain.